1. Field of the Invention
The present invention is related to a printing device such as a printing or copying system employing print heads containing discharging elements, e.g. nozzles, for image-wise forming dots of a marking substance on an image-receiving member, where the marking substance is in fluid form when discharged. Examples of such printing devices are inkjet printers and toner-jet printers. Hereinafter reference will be made to inkjet printers.
2. Description of Background Art
Print heads employed in inkjet printers and the like usually each contain a plurality of nozzles arranged in (an) array(s). The nozzles usually are placed substantially equidistant. The distance between two contiguous nozzles defines the nozzle pitch. In operation, the nozzles are controlled to image-wise discharge fluid droplets of a marking substance on an image-receiving member. When the printer is of the scanning type, the print heads are moveable in reciprocation across the image-receiving member, i.e. the main scanning direction. In such printers, the print heads are typically aligned in the sub scanning direction perpendicular to the main scanning direction. A matrix of image dots of a marking substance, corresponding to a part of an original image, is formed on the image-receiving member by image-wise activating nozzles of the print heads in a traverse of the print heads across the image-receiving member. The printed matrix is generally referred to as a print swath, while the dimension of this matrix in the sub scanning direction is referred to as the swath width. Usually, although not required, the printing swath is constant within a selected printing mode. After a first traverse, when a part of the image is completed, the image-receiving member is displaced relative to the print heads in the sub-scanning direction enabling printing of a subsequent part of the image. When this displacement step is chosen equal to a swath width, an image can be printed in multiple non-overlapping swaths. An advantage of such approach is high productivity, since only a single traverse or printing stage is employed. However, image quality may be improved by employing printing devices enabling the use of multiple printing stages. In the background art, two main categories of such printing devices can be distinguished, i.e. so-called “interlace systems” and “multi-pass systems”.
In an interlace system, the print head contains N nozzles, which are arranged in (a) linear array(s) such that the nozzle pitch is an integer multiple of the printing pitch. Multiple printing stages, or so-called interlacing printing steps, are required to generate a complete image or image part. The print head and the image-receiving member are controlled such that in M printing stages, M being defined here as the nozzle pitch divided by the printing pitch, a complete image part is formed on the image-receiving member. After each printing stage, the image-receiving member is displaced over a distance of M times the printing pitch. Such a system is of particular interest because it achieves a higher print resolution with a limited nozzle resolution.
In a “multi-pass system”, the print head is controlled such that only the nozzles corresponding to selected pixels of the image to be reproduced are image-wise activated. As a result, an incomplete matrix of image dots is formed in a single printing stage or pass, i.e. one traverse of the print heads across the image-receiving member. Multiple passes are required to complete the matrix of image dots. The image-receiving member may be displaced in the sub scanning direction in-between two passes.
Both “interlace systems” and “multi-pass systems” as well as combinations thereof share the advantage of an improved image quality but also the inherent disadvantage of a lower productivity because multiple printing stages are required to render an image part. In practice, the majority of print jobs are executed in such multiple printing stage mode on a scanning type bi-directional printing system, i.e. a printing system capable of printing on the image-receiving member during reciprocation in the main scanning direction.
Such systems are known to be sensitive to gloss variations. Gloss variations can occur when at least a part of the image dots of a marking substance of the same or a different process color are deposited in multiple printing stages in superimposition or at least partially overlapping and when the drying time of the image dots printed on the image-receiving member interacts with the time period required to render all pixels of an image part, i.e. the time period required to complete a sequence of printing stages defined by the print mask. The so-called print mask contains the information about the number and sequence of printing stages and defines which nozzles can be image-wise activated, or in other words contains the information that defines which pixels will be rendered by which nozzles for each printing stage, such that when all printing stages are completed, all the pixels of the image part concerned are rendered. A print mask is associated with a printing mode. Selecting a printing mode enables the user to exchange image quality for productivity and vice versa depending on his requirements. By selecting a printing mode, the nozzles on the print heads, which may be effectively used, are determined as well as the displacement step in the sub scanning direction after each printing stage.
It is known to reduce gloss variations by configuring print masks, which ensure that each position in the sub scanning on the image-receiving member where the image part is to be rendered is exposed to respective printing stages in the same sequence. For instance, suppose a print mask defining four printing stages having, e.g. a sequence 4, 3, 2, 1 is used, each followed by a displacement step of 25% of the swath width. This means that there are positions in the sub scanning on the image-receiving member where the image part is to be rendered that are subjected first to printing stage 4 and subsequently to printing stages 3, 2, 1, while there are also positions in the sub scanning direction on the image-receiving member where the image part is to be rendered that are subjected first to printing stage 3 and subsequently to printing stages 2, 1, 4. For instance, printing stages 4 and 2 may correspond to a traverse of the print head from the left to the right, and by consequence printing stages 3 and 1 correspond to a traverse from the right to the left. It is therefore clear that, although superimposed or partially overlapping image dots on the image-receiving member are deposited in the same sequence, the time intervals between the deposition of the respective image dots are clearly position dependent. This is believed to cause significant gloss variations in the printed images.